I just stumbled over the tag highway=mini_roundabout, see OSM Wiki. As far as I can see GraphHopper doesn't use this tag so far. The tricky thing about this is that it's tagged as a node in the middle of an intersection.

Is there a reason why we don't use this, except for "it's complicated to implement and the gain is not that big"? I couldn't find any issues or other topics about this. I think this would improve the turn instructions, but also mini roundabouts seem to be only used in some minor back roads.

What is the difference between normal roundabout, this and junction=circular?

junction=circular is a roundabout where the traffic in the roundabout has no right of way (very seldom in Germany), junction=roundabout are regular roundabouts, highway=mini_roundabout is a roundabout, but vehicles can drive over the isle in the middle. This is important because big trucks and buses can easily cross these roundabouts. So usually for a mini_roundabout the isle is only paint or a small bump.

No, unfortunately not. These are nodes and no ways. The solution would be probably to create a virtual mini roundabout in the graph as ways and route over these ways or maybe to mark it as a roundabout in some structure similar to turn restrictions.

That is why I mentioned above "it's complicated to implement and the gain is not that big". I think it's not a straightforward feature.